Anaplasma marginale is a Gram-negative obligate intracellular bacterium and the etiologic agent of bovine anaplasmosis, a debilitating infection that is transmitted biologically by ticks, mechanically via fly bites or blood-contaminated fomites, and vertically from mother to calf. It is a febrile illness, the symptoms of which can include anemia, weight loss, abortion, decreased milk production, and death. Due to these clinical manifestations, its propensity to become a chronic infection, and the costs associated with treatment, bovine anaplasmosis results in a combined economic loss for the United States and South American cattle industries that exceeds one billion dollars annually. In sub-Saharan Africa, where livestock sustain the livelihood of the rural poor, the disease can have devastating socioeconomic impacts. A. marginale is a member of the family Anaplasmataceae, which consists of veterinary and human obligate intracellular bacterial pathogens that reside within host cell derived vacuoles. A. marginale predominantly infects erythrocytes in vivo. Detection of the bacterium colocalizing with the endothelial cell marker, von Willebrand factor, in tissue sections from an experimentally inoculated calf indicate it is also capable of infecting endothelial cells in vivo and might serve as a reservoir for infection. Moreover, endothelial cell lines are useful for studying A. marginale infection in vitro, as they are the only mammalian cell type in which continuous cultivation of these microbes has been achieved. The immortalized tick cell line, ISE6, is susceptible to A. marginale infection and supports its replication, making it a useful model for studying bacterial-tick cell interactions.
The pathogen exhibits a biphasic developmental cycle in which it transitions between an infectious dense-cored (DC) form that mediates binding and entry and a non-infectious reticulate cell (RC) form that replicates by binary fission inside the A. marginale-occupied vacuole (AmV). Following replication, RCs reconvert to DCs that exit to invade naive host cells and thereby initiate new infections. Because A. marginale is an obligate intracellular bacterium, adhesins that mediate binding and entry into host cells are essential for survival. Such key virulence factors, however, are poorly defined.
A. marginale expresses the surface protein, OmpA (outer membrane protein A; AM854 in the St. Maries strain), during infection of cattle. OmpA is highly conserved among A. marginale sensu stricto strains and isolates, exhibiting 99.6 to 100% identity. Recent studies demonstrated the importance of OmpA proteins to cellular invasion by A. phagocytophilum (Aph) and Ehrlichia chaffeensis, two Anaplasmataceae members that cause potentially fatal infections of humans and animals. Indeed, it was discovered that A. phagocytophilum OmpA (ApOmpA) is one of a trio of adhesins that cooperatively function to mediate optimal bacterial binding to and invasion of host cells. However, the precise role of A. marginale OmpA (AmOmpA) in Anaplasmataceae infections has yet to be determined.
A. marginale subsp. centrale is used as live vaccine against bovine anaplasmosis in some parts of the world, but this results in unreliable protection as immunity is not uniform against all strains and outbreaks have occurred in immunized populations. Moreover, it is not USDA-approved, has a high production cost, and carries the risks of vaccine-induced disease and transmission of known and unknown pathogens.
Therefore, the need remains for compositions and methods to rapidly and accurately diagnosis new cases and to provide adequate vaccination against Anaplasmataceae infections that cause bovine anaplasmosis.